Typically, in a wireless communications system, the base stations are powered on and continuously operated in an active mode of operation. In this active mode of operation, the base station is operated in accordance with a downlink timing and frequency structure, e.g., a repetitive timing and frequency structure. Synchronization signals, such as beacon signals and pilot signals, are transmitted on a scheduled basis at associated predetermined power levels. The power levels and rate of transmission of these synchronization signals do not typically vary regardless of the number and/or state of users being currently serviced by the base station. In high population density cellular coverage areas, this in not a significant consideration, as there is usually at least one or more active users at any given time using the base station as their network point of attachment and communicating user data. Those active wireless terminals need the full level of synchronization signals such as to maintain precise timing synchronization and maintain accurate current channel estimates.
However, in some cellular coverage areas, such as remote rural areas having low population densities and/or areas having widely varying load requirements as a function of time or a schedule, it would be advantageous if methods and apparatus were developed which allowed a base station to be operated, at certain time and/or under certain conditions, such as to reduce transmission power and/or reduce interference generated by the base station. For example, consider that a base station, e.g., a base station along a train track in a rural area, may have significant time intervals where the base station does not have any registered wireless terminals that need to communicate user data, e.g., receive and/or transmit user data. Under such a situation, during such a time interval the base station power is wasted by transmitting the full set of synchronization signals at the normal power levels. In addition, neighboring cells, which may have high population densities and typically have many active users, will be adversely affected by the interference generated from the unnecessary synchronization broadcast signaling. By reducing the interference level experienced in an adjacent cell the data throughput in that adjacent cell can be increased, e.g., by being able to increase the coding rate for a given transmission power level and modulation scheme.
It would be desirable if methods and apparatus were developed which allowed for reducing broadcast synchronization signals in response to changing system conditions. It would be beneficial if such methods and apparatus supported at least some of: rapid transitioning back to a full level of synchronization signals when required, easily detectable reactivation signaling, seamless hand-off operations, and the capability to transition between different levels of synchronization signaling as a function of schedule information. It would also be advantageous if the methods and apparatus developed to support multiple levels of synchronization signaling would still be capable of supporting registered wireless terminals in a wireless terminal sleep state irrespective of the level of synchronization signaling. In addition, it would be beneficial if the low level of synchronization signaling still provided a wireless terminal with the capability to be able to detect the presence of a base station and/or compare the base station's received signal strength with other adjacent base stations which could potentially be used as network attachment points.
In view of the above, there is a need for new methods and apparatus to implement and support multi-mode base station operations.